Flux

P&I BRAZING FLUX

Molten brazing alloys require a clean surface on which to flow if they are to bond properly. Heating metal during the brazing process causes oxides to form on its surface. These oxides, if not removed, will prevent the filler metal from flowing completely. A coating of flux applied prior to heating the joint will protect the area from exposure to the air and absorb oxides, which may form on the surface.

Because different metals produce different oxides, several flux formulations have been developed to handle these variations. Specialized fluxes are available for both low- and high-temperature brazing of silver, aluminum, nickel-silver and bronze, as well as many soft-solder applications.

FLUX APPLICATION

Flux can be applied in any number of forms including flux paste, powder or pre-made brazing pastes that combine flux with filler metal powder. Flux can also be applied using brazing rods with a coating of flux, or a flux core. In either case, the flux flows into the joint when applied to the heated joint and is displaced by the molten filler metal entering the joint. Excess flux should be removed when the cycle is completed as flux left in the joint can lead to corrosion, impede joint inspection and prevent further surface finishing operations. Fluxes are generally selected based on their performance on particular base metals. To be effective, the flux must be chemically compatible with both the base metal and the filler metal being used. As a general rule, longer brazing cycles should use less active fluxes than short brazing operations.